Good job, Tom....that pretty much sums it all up.
Question 1 : if you missed a spot, just sand the rough areas down and then give the entire part a general "once over" with some finer grit paper to scuff and re-shoot. You can do multiple coats of powder and not be afraid of it. The guys here do multiple coats all the time with great success all the time with little to no trauma.
Question 2 : There's quite a bit of surface area in headers believe it or not. Don't let the looks fool you. What you are dealing with is called lineal inches as opposed to a square inch. One inch of "imagined" coating surface on a flat panel is 1" X 1". Now.... 1" lineal of imagined tube (take 1" of tube and split it down the middle and bend it flat) from a 2 1/2" header is actually 7 sq. inches! See how that can be deceiving? Just to be on the safe side, 2Lbs. of powder would be better that 1 in this case. (and cheaper by the pound when you buy it like that might I add)
Question 3 : Generally it's not acceptable to coat one side and then try it on the other when coating and let the finished side touch heated metal. People here do thier best to either hang, or make a jig for the part to get the most coverage in one pass that they can. I don't know if you are referring to your headers or not, but I'd try to take the flat on the collector tube and the flat from the header flange and work something that way. Maybe a piece of threaded rod or something attached to the oven rack and you fix it into position some how? It's going to be trial and error on that one. Find the way that best suits you and what you are comfortable with in handling. There is ALWAYS a way to get from "A" to "B"...it just needs to be discovered
All in all... a lengthy way of saying "yeah....what Tom said", lol. Hope that helps ya bud.....OH! and welcome to the forum
..... Russ